The present invention relates to the drying and preheating of particulate (usually granular or pulverulent) coking coal.
For reasons known to those skilled in the art, coke is being made more and more often from comminuted coal which, after comminution, is available as a mix of different-size fractions and in moist condition. The comminuted coking coal must be dried; moreover, it has been found that coking conditions can be improved if the coal is furnished to the coking ovens not only in dry condition but in positively preheated condition.
Equipment for effecting such drying and preheating is known, in form of single-stage but usually dual-stage flight stream tubes. These are upright tubes into the lower end of which a stream of hot carrier gas is admitted, in which the coal particles to be dried and heated are entrained. The carrier gas is usually produced in a combustion chamber and mixed with recirculated vapors. The stream issues from the upper end of the flight stream tube and passes through one or more cyclones in which the carrier gas is separated from the coal particles.
The moisture to be expelled from the coal particles usually amounts to about 10% by weight and the subsequent preheating of the coal particles is desired to a temperature of about 200.degree. C. These requirements, especially the preheating, could previously be met in a single-stage flight stream tube only on condition that the incoming carrier gas was very hot (disadvantageous, because it adversely influences later coking characteristics of the coal) and that the flight stream tube was very long (drawback: very high installations with concomitant expense and possible space problems).
A single-stage flight stream tube has been proposed which avoids these problems, in that the coarser coal fractions are temporarily separated from the gas stream and from the finer fractions during movement through the tube, and are then readmitted into the gas stream. This causes the coarser particles to undergo renewed acceleration and improves heat exchange between them and the gas stream as well as the finer particles. While this apparatus and the method practiced with it are valuable improvements over the art prior thereto, still further improvements nevertheless are found to be desirable.